The present invention relates in general to infrared (IR) sensing devices and more particularly to IR sensing devices including circuitry configured to protect the operation of the IR sensing device in an electrically hostile environment.
IR sensors are used to measure temperatures of remote objects by detecting the infrared radiation emitted from the target object. In a typical configuration, IR radiation enters the housing in which the sensor element is located through an IR transparent window and impinges upon the sensing element. The temperature is typically measured by detecting the IR radiation and determining its effect on a thermally isolated radiation absorbing area of the sensing element. The sensing element produces an electrical signal representing the temperature of the target object and which varies with the impinging radiation. This electrical signal is processed and amplified by electronic processing circuitry. The signals are small in amplitude and the sensor is therefore susceptible to the effects of EMC and ESD and also to leakage in protection circuitry.
A typical arrangement of the electronic processing and protection circuitry coupled to a sensing element is shown in FIG. 1. A thermal sensing element 101 is connected to processing circuit 110 via connection lines 112 and 113 as shown. Processing circuit 110 amplifies and processes the signal from sensing element 101 so as to produce a temperature readout signal 107. Sensing element 101 is also connected to protection circuit elements 102, 103, 104 and 105 via connection lines 112 and 113 as shown. Typically, sensing element 101 has one end connected to a ground, or earth, terminal shown here as connection 106. Sensing element 101 typically includes a number of thermopile elements in series with a combined series impedance on the order of tens of [Kohm] kxcexa9. The connections 112 and 113 between the two ends of the series of thermopile elements and the processing circuit are normally protected at one or even both ends against static damage and incoming EMC radiation by protection circuits 102, 103, 104, and 105, which are arranged such as to limit the voltage excursions of the connections using diode structures connected to the power supply rails. The diodes are typically configured to conduct when the voltage on the pins exceeds the power rail voltage by the diode voltage.
Such protection structures, even when not operating, have an inherent leakage current, and when the external impedance is high, e.g., on the order of tens of [Kohm] kxcexa9, the leakage current generates an error voltage superimposed on the desired signal voltage generated by the sensor.
It is therefore desirable to provide IR sensing devices including circuitry that protects the sensing device from electrical interference in an electrically hostile environment and which has reduced leakage.
The present invention provides infrared (IR) sensing devices including circuitry configured to protect the sensing device from electrical interference in an electrically hostile environment and which has reduced leakage.
According to the invention, an IR sensing device includes a sensing element configured as a three terminal device, a differential signal processing circuit and an electrical connection to the conductive housing.
According to an aspect of the present invention, an Infra Red (IR) sensing device is provided that typically comprises an IR sensing element for detecting IR radiation, wherein the IR sensing element includes a plurality of thermopile elements with connections to both ends and to its center point, a processing circuit coupled to the IR sensing element configured to receive and analyze the electrical signals generated by the IR sensing element, wherein the processor connects the center point of the sensing element to a first potential and analyzes the signals from the ends of the sensing element as a differential pair of signals. The device also typically includes protection circuitry associated with the inputs to the processing circuit from the sensing element that connect only to the inputs and the first potential, and an electrically conducting housing connected to the first potential.
According to another aspect of the present invention, an Infra Red (IR) sensing device is provided that typically comprises an IR sensing element for detecting IR radiation, wherein the IR sensing element includes a plurality of serially connected thermopile elements, a processing circuit configured to receive and process the electrical signals generated by the thermopile elements, the processing circuit having first and second inputs coupled to the two ends of the series of thermopile elements and a third input coupled to a center point of the series of thermopile elements and to a first potential, and protection circuitry coupled to the two inputs and the first potential. In operation, the processing circuit typically processes the signals at the first and second inputs as a differential pair of signals relative to the first potential so as to produce a temperature readout signal.
Reference to the remaining portions of the specification, including the drawings and claims, will realize other features and advantages of the present invention. Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with respect to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements.